TP0453, a concealed outer membrane protein of Treponema pallidum, enhances membrane permeability

The outer membrane of Treponema pallidum, the non-cultivable agent of venereal syphilis, contains a paucity of protein(s) which has yet to be definitively identified. In contrast, the outer membranes of gram-negative bacteria contain abundant immunogenic membrane-spanning beta-barrel proteins mainly involved in nutrient transport. The absence of orthologs of gram-negative porins and outer membrane nutrient-specific transporters in the T. pallidum genome predicts that nutrient transport across the outer membrane must differ fundamentally in T. pallidum and gram-negative bacteria. Here we describe a T. pallidum outer membrane protein (TP0453) that, in contrast to all integral outer membrane proteins of known structure, lacks extensive beta-sheet structure and does not traverse the outer membrane to become surface exposed. TP0453 is a lipoprotein with an amphiphilic polypeptide containing multiple membrane-inserting, amphipathic alpha-helices. Insertion of the recombinant, non-lipidated protein into artificial membranes results in bilayer destabilization and enhanced permeability. Our findings lead us to hypothesize that TP0453 is a novel type of bacterial outer membrane protein which may render the T. pallidum outer membrane permeable to nutrients while remaining inaccessible to antibody.     

Assessment of cell-surface exposure and vaccinogenic potentials of Treponema pallidum candidate outer membrane proteins

Syphilis, a sexually transmitted infection caused by the spirochetal bacterium Treponema pallidum, remains a global public health problem. T. pallidum is believed to be an extracellular pathogen and, as such, the identification of T. pallidum outer membrane proteins that could serve as targets for opsonic or bactericidal antibodies has remained a high research priority for vaccine development. However, the identification of T. pallidum outer membrane proteins has remained highly elusive. Recent studies and bioinformatics have implicated four treponemal proteins as potential outer membrane proteins (TP0155, TP0326, TP0483 and TP0956). Indirect immunofluorescence assays performed on treponemes encapsulated within agarose gel microdroplets failed to provide evidence that any of these four molecules were surface-exposed in T. pallidum. Second, recombinant fusion proteins corresponding to all four candidate outer membrane proteins were used separately, or in combination, to vaccinate New Zealand White rabbits. Despite achieving high titers (>1:50,000) of serum antibodies, none of the rabbits displayed chancre immunity after intradermal challenge with viable T. pallidum.